Photo-detectors are used in a wide variety of applications including imaging. A specific type of photo-detector sensitive to the infra-red wavelengths of light is also known as an infra-red detector. Infra-red covers a broad range of wavelengths, and many materials are only sensitive to a certain range of wavelengths. As a result, the infra-red band is further divided into sub-bands such as near infra-red defined conventionally as 0.75-1.4 μm; short wavelength infra-red defined conventionally as 1.3-3 μm; mid wavelength infra-red defined conventionally as 3-8 μm; and far infra-red defined conventionally as 15-1,000 μm. Infra-red in the range of 5 μm to 8 μm is not well transmitted in the atmosphere and thus for many infra-red detection applications mid-wavelength infra-red is referred to as 3-5 μm.
Infra-red detectors are used in a wide variety of applications, and in particular in the military field where they are used as thermal detectors in night vision equipment, air borne systems, naval systems and missile systems. Highly accurate thermal detectors have been produced using InSb and HgCdTe p-n junction diodes, however these thermal detectors require cooling to cryogenic temperatures of around 77 K which is costly. Examples of these existing technologies are presented in FIG. 5A to FIG. 5F. The cryogenic temperatures primarily are used to reduce the dark current generated in the p-n junction diode by among other effects Shockley Reed Hall (SRH) generation.
There are three main contributions to the dark current, denoted as Idark, of photodiodes based on narrow band gap semiconductors. The fluctuations of the dark current components are a major factor in the noise that limits the device performance. These components are:                a) a generation current associated with the Shockley-Reed-Hall (SRH) process in the depletion region, Isrh;        b) a diffusion current associated with auger or radiative processes in the extrinsic area, Idiff; and        c) a surface current associated with the surface states in the junction, Isurf. The surface current depends primarily on the passivation process done for the device. Thus, Idark can be expressed as:Idark=Isrh+Idiff+Isurf  Equation 1        
The SRH generation process is very efficient in the depletion region of photodiodes where the mid-gap traps are highly activated. It is the main source of the dark current in photodiodes operable for mid-wavelength infrared at temperatures below 200K. The current associated with this source is:
                              J          SRH                ≈                  q          ⁢                                          ⁢                                    n              i                                      τ              SRH                                ⁢                      W            dep                                              Equation        ⁢                                  ⁢        2                            where ni is the intrinsic concentration of the semiconductor, Wdep is the depletion width (typically in the range of 1 μm), and τSRH is the SRH lifetime of minority carriers in the extrinsic area. The SRH lifetime of minority carriers in the extrinsic area depends on the quality of the material, i.e. the trap concentration, and is typically in the range of ˜1 μsec in low doped material (˜1016 cm−3). The dependence of SRH current on ni produces an activation energy of        
                    E        g            /      2        ⁢          (                        n          i                ~                  exp          (                      -                                                            E                  g                                2                            kT                                )                    )        ,                 because the source of this generation process is through mid-gap traps. A secondary source of dark current in photodiodes is thermal generation in the neutral regions and diffusion to the other side of the junction. This thermal generation current depends on the auger or radiative process in this area, and is expressed as:        
                                          J            diff                    ≈                                    qp              n                        ×                          1                              τ                diff                                      ×            L                          =                  q          ×                                    n              1              2                                      N              d                                ×                      1                          τ              diff                                ×          L                                    Equation        ⁢                                  ⁢        3                            where τdiff is the lifetime, and in an n-type material exhibiting a doping concentration, denoted Nd, of ˜1−2·1016 cm−3 is in the range of ˜0.5 μsec, depending only slightly on temperature. L is the width of the neutral region of the device or the diffusion length of minority carriers (the smaller of the two) and pn is the hole concentration in the active n type semiconductor in equilibrium and it equal to ni2/Nd. The activation energy of the diffusion current is Eg,        
  (            n      i      2        ~          exp      (              -                              E            g                    kT                    )        )                 as the process involves band to band excitation.        
Additionally, p-n junction diodes, and particularly those produced for thermal imaging require a passivation layer in the metallurgic junction between the p and n layers. Unfortunately this is often difficult to achieve and significantly adds to the cost of production.
There is thus a long felt need for a thermal imaging device that uses a photo-detector with reduced dark noise. Preferably the photo-detector would be sensitive to the mid wavelength infra-red band and not require expensive passivation in production. Further preferably the photo-detector would be operable at significantly higher temperatures than 77K. Further preferably the thermal imaging device would be able to operate for longer periods, be lighter and require less power, when compared to the existing technology in the art.